Incidental to the development of a strain typing set for Streptococcus sanguis, 11/58 strains proved able to inhibit the growth of between 5/10 and 8/10 strains of S. mutans. Limited oxygen and anaerobic environments, both with elevated CO2, were employed. Proposed research will begin with mutagenesis designed both to enhance and eliminate antagonism. Evaluation of in vitro inhibition will be carried out in germfree rats. Relative and absolute bacterial counts, plaque scores and caries incidence will be assayed for individual tooth surfaces and fissures. A pilot study demonstrated the feasability of developing a strain typing system for S. mutans by the use of bacteriocin or bacteriocin-like inhibition. From 500 isolates of group B streptococci, in excess of 100 are promising candidates as antagonists against S. mutans. Challengers which inhibit from 30% to 70% of the indicators are available. A stab-agar method is used. Raw data are entered into a data bank, which, in conjunction with a computer code, will aid in selection of a minimum typing set.